Agronomic Services — News Release

Fertilizing your crop without a current soil test report

by Wayne Nixon, NCDA&CS regional agronomist

HERTFORD — From October through February, northeastern North Carolina received nearly 18 inches of rain. In many areas, waterlogged soils have prevented growers from taking soil samples. With this situation in mind, here are some pointers on how to use soil test reports from the previous year or two to make good nutrient management decisions.

Many growers want to know whether lime is needed. This can partly be answered by comparing the soil pH on a report with the target soil pH. For most field crops, the target pH value should be 6.0 on mineral soils, 5.5 on mineral-organic, or 5.0 on organic soils. Cotton requires a 6.2 on mineral soils.

Recent soil reports, lime application records, and cropping practice history can provide clues about whether fields need liming. Consider the lime and fertilizer history of the field, as well as soil type. Normally, crops will grow well in fields that have been limed according to the soil test recommendations within the last two years.

However, if you have applied high rates of nitrogen, additional lime may be necessary. Nitrogen sources, except for nitrate, are very acid forming and tend to lower soil pH. Changes in pH also occur more rapidly in sandy, low cation exchange capacity soils (CEC < 4) than in fine-textured soils.

Soil test reports from recent years also provide good clues as to what your current fertilizer rates should be. Since phosphorus (P) is held tightly in most mineral soils, its movement is limited unless the soil itself moves. Therefore, phosphorus recommendations should not change much from year to year.

Unlike phosphorus, potassium (K) washes readily out of sandy soils with low cation exchange values (CEC < 4). However, if your soil report shows K-index (K-I) values of 70 or greater and you have been making annual applications, it is unlikely that you will need to apply any potassium for corn, cotton, or soybeans.

Sulfur also washes out of soils easily. Therefore, levels are often critically low on deep, sandy soils. Annual applications of 20 pounds of sulfur per acre may be needed for grain crop production, especially on very sandy soils.

If soils dry substantially in the next few weeks, you may still be able to take soil samples and get your report in time to make decisions about fertilizer and lime. Current turn-around time at the soil testing lab is about three to four weeks and improving every day.

If soils do not dry soon, follow the guidelines given here. For more information, growers in Bertie, Camden, Chowan, Currituck, Gates, Hertford, Pasquotank and Perquimans counties can call me — NCDA&CS regional agronomist Wayne Nixon — at (252) 426-7210. Growers in other counties should visit www.ncagr.com/agronomi/rahome.htm to identify the regional agronomist for their area.